• What is thermal bridging?
• Causes of thermal bridging
• Calculating total heat loss
• Preventing thermal bridging
• How Recticel can help

In this blog, we'll cover everything you need to know about thermal bridging and how it affects thermal performance. We'll talk through what it is, how it occurs, why it matters and our top tips to prevent it in flat roof applications and build-ups.

Thermal bridging occurs when heat escapes via building elements that have higher thermal conductivity than surrounding materials. Also known as cold bridges, these aspects of a building envelope effectively become shortcuts for heat to escape – and, as we know, heat will always find the path of least resistance to flow through and pass more easily.

When heat bypasses the insulation layer, the thermal performance of the building envelope is reduced, sometimes accounting for as much as 30% of total energy loss. More simply, then, a thermal bridge is a weak spot in the construction where heat is lost more easily.

This not only leads to lower energy performance and higher heating bills, but can also create cold spots that contribute to surface condensation, mould growth, and consequently damaged internal finishes. So that's why it's always important to do things the correcticel way – first time, every time.

Thermal bridging typically occurs at breaks, interruptions, or inconsistencies in the insulation layer. The most common causes include:

• Structural elements such as steel beams or concrete upstands that span across the insulation
• Junctions between different elements of the building structure (e.g., wall-to-floor, wall-to-roof, around windows and doors)
• Poor workmanship during installation, such as gaps between insulation boards or misalignment
• Penetrations through the insulation layer, including mechanical fixings or roof curbs
• Debris or mortar ‘snots’ bridging cavities

In flat roof build-ups, thermal bridging often happens at parapets, roof edges, and service penetrations – points where insulation is often interrupted or difficult to detail correctly. Thermal bridges can also be caused by inconsistent insulation thickness or insulation boards that are either not fully secured or in proper contact with adjacent boards (often referred to as floating boards).

These areas are critical in flat roof design because they can lead to localised heat loss, condensation risk, and compromised U-values if not carefully addressed in design and installation. Let's take a closer look at the different kinds of thermal bridging.

There are two main types of thermal bridges to consider in building construction.

Repeating thermal bridges

These occur at regular intervals and are part of the design. For example, wall ties, timber studs, or roof fixings. Because they're evenly distributed, their impact is accounted for in U-value calculations.

Non-repeating thermal bridges

Also known as linear thermal bridges, these are localised and typically found at junctions. For instance, where walls meet roofs or floors, or around windows and doors. Non-repeating thermal bridges are not included in U-value calculations, often resulting in significantly higher heat transfer if not addressed.

Even if the insulation appears continuous, geometry changes at corners or point thermal bridges which are localised at junctions can disrupt the insulation envelope and heat flows, resulting in additional heat loss.

Because U-values don't capture non-repeating (linear) thermal bridging, we use psi values (Ψ) to assess the heat loss through junctions. Psi is measured in watts per metre per kelvin (W/mK) and it tells us how much heat escapes along one metre of a junction.

How to calculate total heat loss from linear bridges

To calculate total heat loss from non-repeating or linear thermal bridges, you need to do the following:

Psi value x length of junction = additional heat loss (in watts per kelvin)

If specific psi values aren’t used, then Part L of the Building Regulations applies a default value y-value of 0.20. Y-values (y) are the average of all linear thermal transmittance across a building envelope, expressed as W/m²K.

A default y-value of 0.20 assumes 20% of total heat loss has come from thermal bridging. However, it's important to note that this is often a conservative estimate that doesn’t reflect the actual extent of heat loss at junctions.

As a result, it can contribute to the performance gap – that is to say, the difference between a building’s designed thermal performance and its actual performance once constructed. Assuming psi values default to 0.20, then, can be a missed opportunity to improve energy performance through better detailing.

Lower psi values can be achieved through:

1. Smart, simplified designs. Thoughtful detailing during the design stage and early coordination with architects, energy assessors, and engineers can simplify designs and reduce potential weak points where heat can escape.
2. High-quality installation and workmanship. Installers should be experienced and understand the importance of continuity in the thermal envelope, particularly at junction and service penetrations.
3. A continuous, unbroken insulation layer. Breaks in insulation cause localised heat loss. Designing for continuity, therefore, can significantly reduce thermal bridging.
4. Detailed junction modelling. Using thermal modelling software allows designers and architects to calculate specific psi values at each junction, rather than relying on default figures.

Using accredited construction details, such as those approved under BRE or LABC schemes, is one way to ensure these principles are followed consistently on site and recognised in energy assessments.

Reducing thermal bridging by simplifying designs and improving installation will have a direct impact on a building’s energy performance. Lower psi values mean lower overall heat loss, making it easier to meet TER, DER, and TFEE/DFEE targets under the Standard Assessment Procedure (SAP).

Preventing thermal bridging begins with understanding where and why it occurs. Here are key steps to improve the overall thermal performance of the building envelope.

1. Design for continuity.

Design details should maintain an unbroken insulation layer. This includes packing insulation into eaves and gables and overlapping wall and roof insulation at junctions.

At roof perimeters, particularly parapets, an insulation upstand of at least 25mm thick and 300mm high should be used to prevent heat loss. Similarly, elements like uninsulated roof curbs can act as significant thermal bridges if not properly addressed.

2. Simplify geometry.

The simpler the junctions, the easier it is to achieve thermal continuity. Complex shapes and overhangs increase junction count and make detailing more difficult.

3. Use the right fixings.

For mechanically fixed systems, use thermally broken telescopic tube washers with fixings. They should be positioned near board edges and corners to effectively fix the board in place and so reduce thermal bridging.

4. Ensure airtightness.

Heat doesn’t just escape through solid materials. Uncontrolled air leakage can account for up to 15% of heat loss. That's why airtightness should be considered during the design stage, with clear lines shown on drawings and followed on site.

5. Install insulation properly.

Always use qualified contractors and make sure boards are fitted tightly and correctly, with no gaps or damage. Unavoidable gaps should be filled with expanding foam to prevent heat loss.

Avoid bridging cavities with debris or mortar droppings, and ensure boards are installed flush to internal surfaces to prevent thermal bypass.

6. Choose single-layer insulation systems

Modern single-layer insulation systems are typically preferred over traditional multi-layer approaches. This is because single-layer systems simplify installation, reducing the risk of misalignment or air gaps and helping to maintain a continuous insulation layer.

Preventing thermal bridging is just one part of a broader, more holistic insulation strategy known as the fabric first approach. 

A fabric first approach is a construction strategy that prioritises getting the building envelope right – or correcticel – before considering add-on technologies like renewables or mechanical systems.

What are the benefits?

By optimising thermal insulation, airtightness, and overall energy efficiency through the materials and design of the building fabric itself, designers can maximise thermal performance and reduce thermal bridging.

Fabric first design also helps ensure buildings perform as close to their design as possible. This reduces the likelihood of a performance gap, where as-built energy efficiency falls short of expectations.

A fabric first approach offers several key advantages:

• Improved energy efficiency. Better insulation and fewer thermal bridges results in lower overall heat loss, which helps reduce heating demand and energy bills.
• Enhanced occupant comfort. Consistent internal surface temperatures help eliminate cold spots and draughts, creating a more stable indoor climate.
• Reduced risk of condensation and mould. A continuous insulation layer improves the surface temperature factor, reducing condensation that can lead to mould.
• Regulatory compliance. High-quality fabric performance supports easier compliance with Part L of the Building Regulations, helping to achieve lower TER and DER figures without relying on complex or costly bolt-on solutions.
• Long-term, low-maintenance performance. Trusted building materials like Recticel's PIR insulation boards help maintain thermal resistance over the life of the building with no ongoing maintenance.

To learn more about how a fabric first approach can reduce the performance gap, access our RIBA-certified CPD here.

Thermal bridging across different building elements can compromise energy performance, increase heat loss, and lead to issues such as surface condensation and mould growth. But with the right approach, you can maintain temperature factors, ensure energy-efficient construction, and deliver buildings that perform as expected.

At Recticel, we’re committed to improving the energy efficiency of UK buildings with our range of PIR insulation, which comes backed with comprehensive technical support.

Our flat roof insulation boards are designed to support a fabric first approach, helping contractors and specifiers to reduce thermal bridging and achieve long-term thermal performance. Our Technical Services team are also on hand to support with:

• U-value and psi value calculations
• Junction detailing and product specification advice
• Practical installation guidance, including correct fixings (e.g., thermally broken telescopic tube washers with fixings)

Recticel helps contractors not only meet but exceed UK Building Regulations, while also keeping installation practical and efficient on site. So whether you're working on new build or refurbishment, flat roof systems or complex junctions, we’re here to help you get it correcticel, first time.